The present invention relates to a switching mechanism for a sewing machine with an electronic control which electronically stores, as pattern signals, the amount of needle amplitudes and fabric feeds, drives a control motor per rotation of the sewing machine in response to the pattern signals, outputs the rotation of the control motor via a link mechanism and produces stitching patterns, and more particularly relates to a switching mechanism for the control mount of the electronic control sewing machine.
With respect to the fabric feed amount in the control amount for the formation of stitching patterns by an electronic sewing machine it is practically sufficient in almost all of the patterns to divide the 8 mm between the forward 5 mm and the backward 3 mm into 16 steps and prepare the control for 0.5 mm increment. However in the buttonhole stitching fine feed amounts around 0.25 mm are required, and for this reason it is necessary to divide the 8 mm into 32 steps.
In the electronic control sewing machine which outputs the rotation of the control motor via a turning link, the range of the rotation angle of the turning link is limited to be less than 90.degree. in view of the mechanism. Therefore, for obtaining the fabric feed amounts around 0.25 mm, it is necessary to make the stepping angles of the control motor rotating the turning link less than around 2.8.degree. which is obtained by dividing the 90.degree. into 32 steps.
With respect to the fabirc feed amount in the control amount, for example, as shown in FIG. 1, if the maximum amount W of the needle amplitude is divided into 16 steps, the zigzag pattern as shown may be formed. However, in such patterns where the minimum amount of the needle amplitude is used in one step (in regard to the needle amplitude amount such as (1)-(2)-(3) and (18)-(19)-(20)) in the needle droppings, needle amplitudes smaller than this minimum needle amplitude could not obtained, and the pattern could not be reduced proportionally in the amplitude direction.
In this case, if the maximum amount W of the needle amplitude is determined to be divided into 32 steps, it is possible to proportionally reduce by 1/2, the stitching pattern as shown in FIG. 1 in the amplitude direction, and thus the stitching application will be preferably broadened. Also in this case, the stepping angle of the control motor should be less than about 2.8.degree. for the fabric feed.
For the above mentioned reason, the electronic sewing machine which employs the control motor and outputs the rotation of the motor via the turning link, has conventionally and structurally used a pulse motor of the hybrid type which produces the comparatively small stepping angle. However, this type of pulse motor has the following problems:
In comparison with the pulse motors of other types, inertia of the motor is larger; it takes a long time to determine the positioning; the rotation speed of the sewing machine is restricted; and vibrations, noises and other inconveniences are generated at the time of the slight stepping.
High precision is required to obtain a small stepping angle, so that the motor would be expensive.
Therefore, in the electronic control sewing machine using the turning link as mentioned above, pulse motors of the induction type belonging to PM (permanent magnet) have been reconsidered for the control motor. In comparison with the hybrid type, the inductor type can structurally lessen the inertia of the rotor, and can also be produced at an economical cost. However, it is difficult to apply the inductor type to a electronic control sewing machine, since small stepping angles cannot be obtained as can in the hybrid type.